Carbon nanotubes are excellent in electrical properties and the like. In view of this, various application fields are expected. For example, composite materials obtained by mixing a carbon nanotube with a resin are expected as materials providing excellent properties; however, sufficient effects have not yet been obtained at present. To describe more specifically, even if a carbon nanotube to which no surface treatment is applied is mixed with a resin, the carbon nanotube is only physically in contact with the resin and the addition effect of the carbon nanotube cannot be sufficiently obtained. Accordingly, e.g., introducing a functional group into a carbon nanotube by applying some sort of surface treatment to the carbon nanotube has been investigated. Examples include a carbon nanotube to which a surface treatment using a mixed acid of sulfuric acid/nitric acid is applied. Other examples include a carbon nanotube to which a surface treatment using a mixed acid of sulfuric acid/hydrogen peroxide is applied (Patent Document 1).
Composite materials obtained by dispersing a carbon nanotube in a resin have been expected as materials that can attain high strength and weight reduction. Even in such composite materials, carbon nanotubes to which a surface treatment is applied are used in order to improve the addition effect of the carbon nanotubes. Examples include composite materials obtained by dispersing a carbon nanotube to which a surface treatment using a mixed acid of sulfuric acid/nitric acid is applied in a resin. However, use of a mixed acid of sulfuric acid/nitric acid has a problem in that the mixed acid causes an extremely strong reaction and destroys the graphene layer of a carbon nanotube, with the result that the strength of the carbon nanotube itself decreases. To deal with this problem, it has been reported to employ a method of treating a carbon nanotube with an acid of low concentration (Patent Document 2).